1. Field of the Invention
The present invention relates to a high zoom-ratio zoom lens system that is suitable mainly for use as an imaging optical system for an image sensor, and includes a wide angle range of a half angle-of-view of 35 degrees. The present invention also relates to an electronic imaging apparatus which uses such a zoom lens system.
2. Description of Related Art
In recent years there has been an increasing need for a zoom lens system in a compact digital camera to achieve a wider angle-of-view and a high zoom ratio. Whereas, further advancement in the miniaturization of the pixel pitch has occurred, thereby increasing the demand for the optical system to have a higher resolution. It is very common for imaging lens systems which are mainly used for compact digital cameras to achieve a slimmer camera body by employing a retractable mechanism that uses multi-stage barrels to reduce the air-distance between lens groups when the zoom lens system is fully retracted to the accommodated position. In order to achieve a miniaturized unit (camera body) when the zoom lens system has fully retracted, the zoom lens system is required to have thin lens groups (small dimensions in the optical axis direction) as well as the frontmost lens group having a small lens diameter. Furthermore, at the design stage, consideration needs to be given to mechanical restrictions in regard to the overall length of the zoom lens system. Furthermore, reducing the cost of the zoom lens system is also an important issue.
Zoom lens systems configured of a negative first lens group, a positive second lens group, and a positive third lens group, in that order from the object side, i.e., three lens groups, are known to be used for compact digital cameras (Japanese Patent No. 4,189,257 and Japanese Unexamined Patent Publication Nos. 2009-37125 and 2004-258235). In such a type of zoom lens system, the second lens group is configured of two lens elements, i.e., a positive lens element and a negative lens element, in that order from the object side.
However, in each of the zoom lens systems taught in the above-mentioned Japanese Patent No. 4,189,257 and Japanese Unexamined Patent Publication Nos. 2009-37125 and 2004-258235, the zoom ratio is only approximately 2:1 through 4:1, which does not satisfy the required zoom ratio specifications. Furthermore, since the Abbe number with respect to the d-line of the positive lens element provided closest to the object side within the second lens group and the partial dispersion ratio of this positive lens element are inappropriate, the secondary spectrum of the axial aberration increases, so that correction of chromatic aberration (axial chromatic aberration) over the entire zoom lens system becomes difficult, and this positive lens element is easily influenced by changes in temperature and changes in environment.